System and Method for Hydrogen Sulfide Decontamination

ABSTRACT

The invention describes a system and method for hydrogen sulfide decontamination of natural gas using a scavenging reagent. The system uses a scavenging reagent within two reactors wherein the consumption of scavenging reagent is optimized by the control of flow of clean and partially-consumed scavenging reagent within and between the two reactors.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 120 of U.S. Provisional Patent Application No. 60/981,333 filed Oct. 19, 2007, the entire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The invention describes a system and method for hydrogen sulfide decontamination of natural gas using a scavenging reagent. The system uses a scavenging reagent within two reactors wherein the consumption of scavenging reagent is optimized by the control of flow of clean and partially-consumed scavenging reagent within and between the two reactors.

BACKGROUND OF THE INVENTION

As is known, hydrogen sulfide (H₂S) is a highly poisonous and corrosive contaminant of natural gas and crude petroleum. While only relatively small amounts of H₂S occur in crude petroleum, natural gas can contain up to 40% by volume. As a result, H₂S must be removed to acceptable levels prior to delivery to the refinery or main gas distribution system. Generally, in order to meet governmental, technical and natural gas sales specifications, H₂S concentrations must be at very low levels (usually less than 16 ppm).

Hydrogen sulfide is a covalent hydride structurally related to water (H₂O) as oxygen and sulfur occur in the same periodic table group. However, hydrogen sulfide is weakly acidic, dissociating in aqueous solution into hydrogen cations H⁺ and the hydrosulfide anion HS⁻:

H₂S→HS⁻+H⁺

Hydrogen sulfide reacts with many metals cations to produce the corresponding metal sulfides.

In petroleum refineries, the normal hydrodesulfurization processes liberate sulfur from petroleum by the action of hydrogen. The resulting H₂S is converted to elemental sulfur by partial combustion via the Claus process, which is a major source of elemental sulfur.

The most highly utilized processes for sweetening sour natural gas is to use amine solutions to remove the hydrogen sulfide. These processes are known simply as the ‘amine processes’, or alternatively as the Girdler process, and are used in 95 percent of North American gas sweetening operations. Generally, the sour gas is run through a tower, which contains the amine solution. This solution has an affinity for sulfur, and absorbs it much like glycol absorbing water. There are several amine solutions that are commonly used, including monoethanolamine (MEA), methyldiethanolamine (MDEA), and diethanolamine (DEA) each of which in their liquid form, will absorb sulfur compounds from natural gas as it passes through the column. The effluent gas or sweet gas is virtually free of H₂S compounds. Like the process for NGL extraction and glycol dehydration, the amine solution used can be regenerated (that is, the absorbed sulfur is removed), allowing it to be reused to treat more sour gas. This technology is capital intensive and is generally more suitable for larger scale operations.

In other systems, the use of liquid scavengers within columns is also known. In these systems, sour gas and a liquid scavenger agent are introduced into a column. The scavenger reacts with sour gas within the column such that both sweet gas and “spent” scavenger are removed from the top of the column. The most common liquid scavenger is an amine-aldehyde condensate manufactured by an exothermic reaction of monoethanolamine and formaldehyde. Water and methanol are usually required to keep the formaldehyde in solution and prevent polymerization. The resulting “scavenger” product is a hexahydrotriazine, and is commonly called “triazine” in the industry. The “triazine” is typically offered in a water-based solution. In most applications, the reaction products are also water soluble, with very low toxicity characteristics making this a relatively simple system to handle. Other scavenging reagents are known to those skilled in the art.

Importantly, the scavenging reactions between triazine and H₂S can be “overspent” such that the reaction products are solids. Generally, it is preferred that solid reaction products are not produced for ease of subsequent handling. Thus, most reactions are controlled to underutilize the scavenging reagent.

While the liquid scavenger system is a relatively cost effective system as a result of the relatively low capital cost of equipment, simple logistics, and simple waste treatment, the cost of scavenger reagent is relatively high. Typically, as a result of the cost of the liquid scavenger, the overall process cost of H₂S removal will range from a low of $8/pound to $20/pound of H₂S removed. Notwithstanding the cost of reagent, the liquid scavenger system is a preferred system for offshore gas treatment and onshore sites where there is a relatively small amount of H₂S that needs to be treated.

However, there continues to be a need for a technology that improves the efficiency of utilization of scavenger reagent, such that the overall process economics can be improved.

SUMMARY OF THE INVENTION

In accordance with the invention, there is provided a system and method for improving the efficiency of utilization of scavenger chemical reagent in a sour gas treatment process.

In a first embodiment, the invention provides a system for removing hydrogen sulfide from natural gas comprising: a first reactor for reacting a partially-consumed scavenging reagent with sour natural gas and for producing partially-sweetened natural gas; a separator operatively connected to the first reactor for separating consumed scavenging reagent from the partially-sweetened natural gas; a second reactor operatively connected to the separator for reacting clean scavenging reagent with the partially-sweetened natural gas and for producing sweetened natural gas; a scavenging reagent delivery system operatively connected to the first reactor and second reactor, the scavenging reagent delivery system for delivering clean scavenging reagent to the second reactor and partially-consumed scavenging reagent to the first reactor; and, a control system for controlling the relative flow of scavenging reagent to the first and second reactors in response to the hydrogen sulfide concentration within the partially-sweetened natural gas.

In a further embodiment, the invention provides a method for removing hydrogen sulfide from natural gas comprising the following steps in any order: a) reacting a partially-consumed scavenging reagent with sour natural gas to produce a partially-sweetened natural gas and consumed scavenging reagent; b) separating consumed scavenging reagent from the partially-sweetened natural gas; and, c) reacting clean scavenging reagent with the partially-sweetened natural gas to produce sweetened natural gas; wherein the clean scavenging reagent from step c) is used as partially-consumed scavenging reagent in step a).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described by the following detailed description and drawings wherein:

FIG. 1 is a schematic diagram of a hydrogen sulfide plant and polishing system in accordance with the prior art;

FIG. 2 is a schematic diagram of a hydrogen sulfide processing plant and polishing system in accordance with the invention; and

FIG. 2A is a schematic diagram of a hydrogen sulfide processing plant and polishing system in accordance with an alternate embodiment of the invention.

DETAILED DESCRIPTION

In accordance with the invention and with reference to the figures, embodiments of a system and method for removing hydrogen sulfide from natural gas are described.

The system and method improves the efficiency of scavenger reagent (SR) utilization in typical hydrogen sulfide sweetening processes.

As shown in FIG. 1 and in accordance with the prior art, a typical hydrogen sulfide treatment plant utilizing a scavenger reagent includes a primary reactor (or column) 10 and separator 12. Sour gas 10 a is introduced at a low point 10 b in the column together with SR 10 c from a fresh SR source 14 by pump 11. The sour gas and SR pass upwardly through the column whereby the sour gas is sweetened and the SR is consumed as known to those skilled in the art. The sweetened gas and SR 10 d collectively pass over the top of the column and thereafter enter separator 12 whereby the sweetened gas and liquid SR are separated on the basis of density. The liquid SR is removed from the bottom 12 a of the separator and delivered to a spent reagent tank 16 for disposal and the sweetened gas is removed from the top 12 b of the separator for delivery. The system is controlled by an appropriate control and feed back system 18 to monitor the H₂S concentration in the sweetened gas 12 b and to control the flow of SR to the column 10 through pump 11.

In order for the sweetening reactions to proceed and to ensure that the sweetened gas meets the appropriate regulatory standard for H₂S removal, the SR must be added in significant excess to ensure that the H₂S removal reaction proceeds to completion. As a result, due to normal fluctuations in the H₂S concentration entering the column 10, and to provide an appropriate safety margin, significant amounts of SR delivered to the spent reagent tank 16 may be unreacted.

In accordance with the invention, and with reference to FIG. 2, a system to improve the efficiency of SR utilization is provided. Generally, the primary desulfurization system 10, 12 is used with partially-consumed SR 20 a to produce a “semi-sweet” gas 12 b and clean SR 14 a is used to polish the semi-sweet gas 12 b to produce a sweet gas 20 b. As a result, the system, by virtue of the use of clean SR in the final polishing step enables more effective control of the utilization of SR.

In accordance with the invention, the system as described in FIG. 1 is modified to include a polishing system 20 comprising a second column that functions similarly to column 10 with the exception that it is operated as a combined reactor and separator. In addition, the system introduces clean SR 14 a directly to column 20 prior to introduction into column 10 and the system is controlled such that semi-sweet gas 12 b is introduced into column 20. In addition, the system includes pump 11 a to deliver clean SR to column 20 and the control system 18 is modified to balance the effective flow rates through both pumps 11, 11 a in response to the measured H₂S concentration from separator 12, reagent levels in column 20 as measured by level controller 20 c and in the produced sweetened gas.

Generally, the control system operates to ensure that the H₂S concentration exiting column 20 is low (generally less than 16 ppm, ideally 0 ppm). Primary control of the system is by conducted on the basis of the measured H₂S level between separator 12 and column 20. For example, for a given set of operating parameters (i.e. based on the H₂S levels, system volumes and stoichiometry of the specific system), the system may be designed such that the measured H₂S level in semi-sweet gas 12 b is in the range of 10-100 ppm in order that a desired H₂S level of the sweet gas is at the desired level (ideally 0 ppm). As such, if the control system determines that the H₂S level is within this range, pumps 11 and 11 a will in turn be run at a given flow rate. If the H₂S level is detected to be above this range, indicating a possible spike in H₂S level in the source gas, the control system will increase flow rates through pumps 11, 11 a so as to increase the flow of SR within the columns. Similarly, a decrease in H₂S level below this range, will cause a decrease in flow rates through pumps 11, 11 a so as to reduce the flow of SR in the columns. Readings of H₂S concentrations in the sweet gas 20 b and source gas 10 a may be made for safety purposes and reference points but are generally not required for system control after the system is operating.

By way of representative example, the control system and the balance of SR is described as follows: If the semi-sweet gas 12 b is 95% desulfurized in column 10, the remaining 5% of the H₂S is removed by reacting the semi-sweet gas with clean SR in column 20. The clean SR ensures that the desulfurization reactions in column 20 proceed to effectively 100% completion whilst depleting only 5% of the desulfurization capacity of the specific volume of clean SR. The partially-consumed SR 20 a is introduced into column 10 at a flow rate that ensures the complete utilization of SR to produce semi-sweet gas 12 b. By responding to changes in the H₂S concentration in semi-sweet gas 12 b, the controller 18 can adjust the relative flow rates of SR between columns 10 and 20 and the level of SR within column 20. As a result, the system can be controlled to more effectively ensure complete utilization of SR whilst producing sweet gas. Thus, depleted SR entering tank 16 is fully depleted.

In a further embodiment as shown in FIG. 2A, partially consumed SR 20 a is returned to tank 14 a prior to pumping to column 10. From a practical perspective, this configuration may be preferred in the field particularly if the system is being retro-fit to a system in accordance with the prior art.

As a result, the system is able to effectively utilize SR without the shortcomings of the prior art by specifically being able to fully utilize the SR.

EXAMPLE

A cost comparison between the prior art and the subject process is detailed in Table 1 for a triazine SR under the stated operating conditions. It is understood that specific operating conditions will vary depending on the numerous variables including vessel sizes, operating pressures and temperature and gas source as may be established for or measured at a particular site.

TABLE 1 Cost Comparison at Representative Operating Conditions Operating Conditions Design Pressure 1440 psia Operating Pressure 300 psia Operating Temp 90° F. Gas Flow 1.0 MMscfd H₂S Inlet 2400 ppm H₂S Outlet 0 ppm Scavenging Reagent Triazine Scavenging Rate 0.2 L/ppm/MMscfd (100%) Cost Comparison Parameter Subject Process Prior Art Process System Efficiency 100% 80% Scavenging Rate 0.2 l/ppm/MMscf 0.25 l/ppm/MMscf Daily Chemical Use 480 l/day 600 l/day Cost/Liter 3 $/liter 3 $/liter Daily Chemical Cost 1440 $/day 1800 $/day Process Cost 1.44 $/Mcf 1.8 $/Mcf Changeout/fill frequency 67 Days 53 Days Changeout per year 5.5 Fills/year 7 Fills/year Annual Chemical Cost $525,000/year $657,000 $/year Annual Savings $131,400/year

As shown, it is clear that based on the efficiency of fully using the SR, significant costs savings can be realized with the subject technology for a typical sour gas well.

Although the present invention has been described and illustrated with respect to preferred embodiments and preferred uses thereof, it is not to be so limited since modifications and changes can be made therein which are within the full, intended scope of the invention. 

1. A system for removing hydrogen sulfide from natural gas comprising: a first reactor for reacting a partially-consumed scavenging reagent with sour natural gas and for producing partially-sweetened natural gas; a separator operatively connected to the first reactor for separating consumed scavenging reagent from the partially-sweetened natural gas; a second reactor operatively connected to the separator for reacting clean scavenging reagent with the partially-sweetened natural gas and for producing sweetened natural gas; a scavenging reagent delivery system operatively connected to the first reactor and second reactor, the scavenging reagent delivery system for delivering clean scavenging reagent to the second reactor and partially-consumed scavenging reagent to the first reactor; and a control system for controlling the relative flow of scavenging reagent to the first and second reactors in response to the hydrogen sulfide concentration within the partially-sweetened natural gas.
 2. A system as in claim 1 wherein the control system includes an H₂S sensor operatively connected between the separator and second reactor for measuring H₂S concentration exiting the separator and wherein the control system is responsive to the H₂S concentration exiting the separator to increase or decrease the relative flow of scavenging reagent to the first and second reactors.
 3. A system as in claim 1 wherein the second reactor includes a level controller operatively connected to the control system for controlling the level of scavenging reagent within the second reactor.
 4. A system for removing hydrogen sulfide from natural gas comprising: a first reactor for reacting a partially-consumed scavenging reagent with sour natural gas, for producing partially-sweetened natural gas and separating consumed scavenging reagent from the partially-sweetened natural gas; a second reactor operatively connected to the first reactor for reacting clean scavenging reagent with the partially-sweetened natural gas and for producing sweetened natural gas; a scavenging reagent delivery system operatively connected to the first reactor and second reactor, the scavenging reagent delivery system for delivering clean scavenging reagent to the second reactor and partially-consumed scavenging reagent to the first reactor; and a control system for controlling the relative flow of scavenging reagent to the first and second reactors in response to the hydrogen sulfide concentration within the partially-sweetened natural gas wherein the control system includes an H₂S sensor operatively connected between the first reactor and second reactor for measuring H₂S concentration exiting the first reactor and wherein the control system is responsive to the H₂S concentration exiting the first reactor to increase or decrease the relative flow of scavenging reagent to the first and second reactors.
 5. A system as in claim 4 wherein the second reactor includes a level controller operatively connected to the control system for controlling the level of scavenging reagent within the second reactor.
 6. A method for removing hydrogen sulfide from natural gas comprising the following steps in any order: a) reacting a partially-consumed scavenging reagent with sour natural gas to produce a partially-sweetened natural gas and consumed scavenging reagent; b) separating consumed scavenging reagent from the partially-sweetened natural gas; and c) reacting clean scavenging reagent with the partially-sweetened natural gas to produce sweetened natural gas; wherein the clean scavenging reagent from step c) is used as partially-consumed scavenging reagent in step a).
 7. A method as in claim 6 wherein the flow of scavenging reagent used in steps a) and c) is controlled in response to the hydrogen sulfide concentration within the partially-sweetened natural gas.
 8. A method as in claim 7 wherein scavenging reagent consumption in step a) is effectively 100%. 